If you want to see the effect of script changes on the DOM (that is, you want to see the current document structure instead of the original HTML source) in firefox take a look at firebug. It shows live updates to the DOM and CSS (and allows you to change it), has a script-accessible javascript log (with stack traces), can evaluate user-entered javascript in the current page's context, logs XMLHTTP requests and more. It really is the most useful tool I know for javascript (and HTML) debugging.